Add fedCM to accommodate google migration (#798)

- See migration guidelines here: https://developers.google.com/identity/gsi/web/guides/fedcm-migration#fedcm_flag

src/khoj/interface/web/login.html

@@ -28,7 +28,8 @@
             </div>
             <div id="g_id_onload"
                 data-client_id="{{ google_client_id }}"
-                data-ux_mode="redirect"
+                data-ux_mode="popup"
+                data-use_fedcm_for_prompt="true"
                 data-login_uri="{{ redirect_uri }}"
                 data-auto-select="true">
             </div>